[SunkenCiv]

Periodicity of bombardment has been suggested (as noted above by allmendream), but each time "Nemesis" has been looked for it has not been found, which has caused its hypothetical parameters to be revised. Matese et al rely on comet focussing, that is, a common point of origin of many long period comets, as well as the hypothetical Oort Cloud. At least one other researcher regards comet focussing to be an artifact of having most of the observatories in the northern hemisphere of the Earth.

Impacts happen all the time, but serious ones only once in a while. IMHO the fact is, periodicity is just a dodge to keep impacts from happening at all, an intellectual (rather, quasi-intellectual) method of keeping the Earth, and quite a number of careers, nice and safe from catastrophism.

Impacts happen at random. Far and away the bulk of the impacting objects come from objects also in orbit around the Sun, rather than from interlopers from outside the Solar System. The objects themselves may have common origins here and there (there was a calculation of the origin of the Chicxulub object, which said that most of the rest of the original object remains in orbit around the Sun as space debris), but basically, there are so many objects known and unknown, that the problem becomes too complex for periodicity to work. This explains, I think, the reason for the enormous margins of error in the so-called periods.

Nemesis: Does the Sun Have a 'Companion'?
by Robert Roy Britt
03 April 2001

Richard A. Muller... a physicist at University of California at Berkeley... [has] ideas... generally rooted in solid science and genius extrapolation... But Muller's biggest idea is a real Nemesis. Or so he claims. Like a thorn in the side of mainstream researchers, Muller's Nemesis theory -- that our Sun has a companion star responsible for recurring episodes of wholesale death and destruction here on Earth -- seems to reemerge periodically like microbes after a mass extinction. It's a theory that has many detractors. And it's a theory that has been beaten down and left for dead in the minds of most scientists... Muller's idea for Nemesis came to him 1983. Luis Alvarez, then an emeritus professor of physics at the University of California at Berkeley, and his son Walter had recently put forth the theory that a giant impact had wiped out the dinosaurs... Around the same time, two other researchers had suggested yet another controversial idea, that mass extinctions occurred at regular intervals -- every 26 million years or so. Scientists immediately folded the ideas into a new and breathtaking possibility: Impacts by space rocks were causing massive global species destruction every 26 million years.

Theory of Periodic Mass Extinctions
Frank R. Ettensohn

Why has interest in this hypothesis subsided? There are two reasons. First, there are statistical questions about extinction rates that just cannot be answered. It is not clear whether other hypotheses can fit the empirical data as well as does the hypothesis of periodicity. The distribution of extinction events in time is certainly not random, but there is more than one way to analyze the data. Does the fact that the data appear to fit well to a model of periodicity mean that the events truly are periodic? This question was debated throughout the late 1980s without reaching a resolution.

The second reason for the loss of interest is one not uncommon to science: Here is an intriguing observation that no one knows how to explain. Researchers formulated a number of very interesting astronomical hypotheses to account for the 26-million-year periodicity of extinction. The most famous of these was the Nemesis, or 'death star,' hypothesis, which stated that the sun has a distant companion whose highly elliptical orbit brings it into the Oort Cloud (a swarm of frozen comets orbiting far from the sun) once every 26 million years. During each pass through the Oort Cloud, the companion's gravity would scatter huge numbers of comets, some of which would crash into Earth. The environmental damage caused by these impacts would lead to an elevated rate of extinctions.

[ForGod'sSake]

IMHO the fact is, periodicity is just a dodge to keep impacts from happening at all, an intellectual (rather, quasi-intellectual) method of keeping the Earth, and quite a number of careers, nice and safe from catastrophism.

Heh.

...but basically, there are so many objects known and unknown, that the problem becomes too complex for periodicity to work. This explains, I think, the reason for the enormous margins of error in the so-called periods.

Sortof back where it all started, eh? Although there does appear to be something more than pure randomness at work. Speaking of Muller; an interesting "conversation", typos and all:

1. Cosmic Terrorist

 

 

 

            "Rich, I just got a ridiculous paper from Raup and Sepkoski.  They claim that mass extinctions occur every 26 million years, like clockwork.  They are smart scientists, and I don't see how they could write such nonsense.  I've written them a letter criticising their analysis, but I'd like you to look it over before I send it."  It was November, 1983, and Luis Alvarez was making one of his typical requests. 

 

            I knew it would take a lot of work to do the careful checking that Alvarez expected, and I wasn't looking forward to the job.  I would have to study the "ridiculous paper," and understand it in detail.  Then I would have to do the same for Alvarez's letter.  Alvarez expected me to redo each of his calculations from scratch.  But we  depended on each other for this kind of work.  We knew we could each trust the other to do a thorough job, and neither of us had anybody else whom we could really trust.  We often caught each other making silly mistakes; we didn't mind looking foolish to each other, but neither of us liked to look foolish to the outside world.  So I accepted the task, albeit reluctantly.  I had no reason to suspect that this time it would change my life.

 

            The two paleontologists David Raup and Jack Sepkoski had collected a vast amount of data on family extinctions in the oceans, and their analysis of these data showed that there were intense periods of extinctions, mass extinctions, every 26 million years.  I was already convinced by the earlier work done by Alvarez and his son Walter that the dinosaurs had been killed by an asteroid impact, yet even that mass extinction fell right into the apparently regular schedule.  That seemed to imply that asteroids were causing periodic disasters.  How could Nature arrange asteroids so that one would hit the earth at just the right time, every 26 million years?  It was as if Raup and Sepkoski  hypothesized the existence of some cosmic terrorist, sitting out there with an asteroid gun, aiming at the earth.  It was ludicrous.  

 

            I scanned their paper, only to find that Raup and Sepkoski had no theory, only facts. Of course it wasn't logically necessary to hypothesize that all the periodic extinctions came from asteroid impacts, but we knew that two of them did: the Cretaceous/Tertiary extinctions that eliminated the dinosaurs 65 million years ago, and the Eocene/Oligocene extinctions that killed many land mammals 37 million years ago.  It seemed too far fetched to postulate a different source for all the other mass extinction when these two fit right in their cycle.   The implications of the Raup and Sepkoski work were so ludicrous, that they must have made some mistake in their data collection or in their analysis.   The only challenge was to find their mistake. 

 

            Alvarez thought that he had identified several errors.  I turned to his letter before I finished reading the Raup and Sepkoski paper.  Alvarez argued in this letter that Raup and Sepkoski had failed to prove that there was a true periodicity in the data.  He found that their compilation of mass extinctions had included some events of doubtful statistical significance. In addition, they had kept in their analysis both the Cretaceous/Tertiary and the Eocene/Oligocene events, even though we knew that they were due to asteroid impacts and therefore could not be part of any periodic cycle.  When these two extinctions were removed, the remaining extinctions in their data had such doubtful statistical significance that Alvarez claimed there was no significant periodicity left. 

 

            Alvarez's arguments seemed to make a lot of sense.  Then I finished reading the Raup and Sepkoski paper.  These paleontologists must be wrong, I felt.  But I wouldn't be doing my job for Alvarez unless I looked at it more carefully, so I read their paper again.  I knew Alvarez relied on me because I was not a "yes man."  He didn't want psychological support; he wanted me to find whether anything was wrong with his criticism.  My role was to play devil's advocate, to try to prove that Raup and Sepkoski were right.

 

            I asked myself: If their effect were real, why had nobody discovered it before?  This is the first question one should ask, since for real discoveries it usually has a good answer.  I found one immediately: Raup and Sepkoski had made the most detailed and careful collection of fossil extinctions that had ever been assembled.  They were in a unique position to discover something new.  That fact bothered me.  I hate to dismiss out of hand the work of real experts.  Dave Raup, in particular, had a distinguished career, and had been elected to the Academy of Sciences, a very high honor usually given only to scientists will a long record of excellent work.

 

            I studied the plot they had made that purported to show that the extinctions occured every 26 million-years.  I decided that the plot wasn't convincing,  but was it wrong?  There is a big difference.   I mustn't be too skeptical.  I didn't have the luxury of saying, simply, "I'm not convinced."  The devil's advocate must be an advocate.  Even innocent suspects can be convicted if they have poor lawyers.  I knew that Alvarez depended on me to argue their case in the strongest possible way.  

 

            Physicists have their own conventions for making plots, so I took their data and replotted it, in a way that a physicist would want to see it.  The new plot looked more impressive than I had expected.  It was a rough version of the one shown below:

 

I drew the arrows at the regular 26 million year period intervals.  Underneath them was the extinction data. The arrows seemed to line up rather well with the peaks in the extinction intensity, except for those near 170 Myr and 196 Myr.  With this plot in front of me I found it hard to reject their conclusion out of hand.  I decided that I had better reexamine Alvarez's case, and see if it was flawed.

 

            I found it particularly difficult upon my second reading of his letter to accept Alvarez's argument that the Cretaceous/Tertiary and Eocene/Oligocene extinctions should excluded in the analysis.  What right did he have to assume  that asteroid impacts could not come periodically?  True, I couldn't think of any plausible way to throw the asteroids at the earth on a regular schedule, but maybe I just wasn't clever enough.  Not finding something is not the same as proving it is not there. 

 

            At this point I don't think that I was convinced that Raup and Sepkoski were right, but I felt I had found a flaw in Alvarez's argument.  I went to Alvarez's office and told him that I found a mistake in his letter.  I explained why I felt that it had been improver for him to exclude the the Cretaceous/Tertiary and Eocene/Oligocene mass extinctions from the analysis.  I argued like a lawyer,  interested in proving my client innocent even though I still wasn't totally convinced myself.

 

            Alvarez rejoined strongly, like a lawyer himself.  "To keep those extinctions in the analysis would be cheating, Rich" Alvarez said.  I was thrown off balance by his beligerent offense.  It was hard not to interpret Alvarez's strong language as an attack on me, suggesting that I didn't know how to do honest physics.  I had been Alvarez's student for many years, first as a graduate student and then as a young research physicist working under his guidance, and finally as a professor of physics who still respected the judgement of his Nobel-Prize-winning mentor.  Alvarez continued,  "You are taking a no-think approach.  Scientists aren't allowed to do that."  I bristled a little at his implication that I was not behaving like a scientist.  Alvarez said, "You can't ignore something you already know to be true.  And we know these two extinctions came from asteroid impacts." 

 

            I knew many people who refused to argue with Alvarez because his aggressiveness, or perhaps because they usually lost.  But Alvarez and I had worked together for nearly two decades, and I knew that he respected me and my work.  His accusations were not meant to be personal; they were just his way to be forceful.  I knew he respected me.  I knew he respected me.  I kept on reminding myself of this as I tried to keep my temper, and argue with him, as he expected me to. 

 

            I told Alvarez that my approach was not "no think", but that it is proper to ignore certain "prior knowledge" in testing a hypothesis.  We should not say that the two known impact-caused extinctions could not be part of a larger periodic patter,  since we didn't yet know what was causing the regular extinctions.  Alvarez repeated, without change, his previous argument, but with a little more emphasis on the word "no think."  I sensed that Alvarez was beginning to have trouble controlling his temper.  "How could Rich be so dumb?" he seemed to be thinking.    Not very effectively, I repeated my old arguments.   He knew he was right.  I knew I was right.  We seemed to be talking right past each other.  We weren't getting anywhere.  This was not a question of politics, or religion, or of opinion.  It was a question of data analysis, something all physicists should be able to agree on.  Certainly Alvarez and I should be able to agree. 

 

            I tried again: "Suppose someday I found a way to make an asteroid hit the earth every 26 million years.  Then would you admit that all the data should be included in the analysis, including the Cretaceous and E/O events?"  I thought I had Alvarez on that one. 

 

            "What is your model?" he demanded.  I felt he was evading my question. 

 

            "It doesn't matter!" I replied.  "It's the possibility of such a model that makes your logic wrong, not the existence of any particular model." 

 

            Alvarez retorted, "Look Rich, I've been in the data analysis business a long time.  I think you'll agree that I have had a lot of experience in analysing data.  You cannot take a no-think approach and ignore something you know." 

 

            I began to sense that I really was getting angy. I thought I detected a slight quiver in Alvarez's voice too. Alvarez was asking me to play "devil's advocate," and now he was claiming authority!  Lower your voice, Rich.  Remain calm.  Don't show Alvarez that  you're getting annoyed!

 

            In a calmer voice, I continued.  "I could come up with a model, but I don't think I have to.  Unless you can prove that such models are impossible, it invalidates your logic." 

 

            "What's your model?" he demanded.  At this point my frustration almost brought me to the breaking point.  Why couldn't Alvarez understand what I was saying?  How could he be so stupid? 

 

            Damn it!  O.K., if I have to, I'll win this argument on his terms!  I'll invent a model.  It will serve him right!   After a moment's thought, I said: "Suppose the sun is really part of a binary system, and there is a companion star orbiting the sun in a narrow ellipse.  Every 26 million years it comes close to the earth and does something, I'm not sure what, but it makes asteroids hit the earth.  Maybe it brings the asteroids with it."

 

            I was surprised by Alvarez's thoughtful silence.  He seemed to be taking my idea seriously, and mentally checking to see if there was anything wrong with it.  What had happended to his anger?  Finally he said, in a calm voice, "You surprised me, Rich.  I was sure you were going to come up with a model that brought in dust or rocks from outside the solar system, and then I was ready to hit you with a fact I bet you didn't know.  Frank and Helen  have shown that the iridium came from within our own solar system, by finding that the Rhenium-187/Rhenium-185 ratio in the boundary clay is the same as for rock in the earth's crust.  I figured that you didn't know this fact, since it has never been published.  But you fooled me.  Your companion star was presumably born with the sun, and so it should have the same isotope ratios as the sun.  So the argument I had in reserve was no good.  Nice going."

 

            Alvarez paused, and I stood there silent.  He had been thinking a step ahead of me, anticipating my move, like a chessmaster.  He had guessed what my criticism would be, and had his answer ready, but I had fooled him by making a surprise move.  He acted pleased, as if he was proud that his student could surprise him. He continued, "But I think that your orbit would be too big.  The companion would be pulled away by the gravity of other nearby stars."

 

            I hadn't expected the argument to cool down so suddenly.  We were back to discussing physics, not authority or logic.  I hadn't meant my model to be taken seriously, although I had felt that my point would have be made if the model could withstand assault for at least a few minutes.  I hadn't the rhenium argument, but I got it straightened out later.   Alvarez's son Walter Alvarez had found a clay layer that had been deposited in the oceans precisely at the time that the dinosaurs and two third of all the species on the earth were destroyed.  This clay layer, they hypothesised, had been created by the impact of an extraterrestrial body (such as a comet or asteroid) on the earth. Rhenium comes in several forms, Rhenium-185 which is stable, and  rhenium-187 which is radioactive and it disappears with a half-life of 40 billion years.  In the 4.5 billion years since the formation of the solar system, approximately 8% of the rhenium-187 should have disappeared.  Unless the rhenium in the clay was produced at the same time as the rhenium in the earth (i.e. at the formation of the solar system) the ratios were very unlikely to match those on the Earth as well as they did.

 

            Now I took the iniative.  "Let's calculate how big the orbit would be"  I suggested.  I went over to the blackboard and wrote down Kepler's laws of planetary motion.  The half-diameter of the orbit would be the period 26,000,000 years, raised to the 2/3 power.  I pushed the buttons on my Hewlett-Packard 11C pocket calculator, and had the answer: 88,000 astronomical units, i.e. 88,000 times as far as the distance from the earth to the sun, about 1.4 light years.  (A light year is the distance that light travels in one year.)  The distance of the star from the sun, assuming a very eccentric orbit, should be roughly twice this, just under three light-years.  That put the companion star substantially closer to the sun than any other star.  It would not get pulled away by other stars, as Alvarez had suggested.  Alvarez agreed with the calculation.  The theory had survived five minutes, so far.

 

            "Okay, I was wrong.  It looks good to me."  Alvarez's turnaround was as abrupt as his argument had previously been fierce.  He seemed to be taking my model seriously, even more seriously than I was taking it.   I didn't really believe the Raup and Sepkoski data at that time.  I needed more time to study it carefully.  I had just felt that  Alvarez had dismissed it too readily. 

 

            It was my turn to say something nice to Alvarez.  But he spoke first: "Let's call Raup and Sepkoski and tell them that you found a model that explains their data."  Alvarez continued to surprise me. 

 

            I felt bemused and aloof as I followed him to his office for the phone call to  the University of Chicago.  Raup was away, but someone took a message.  Alvarez said he would let me know as soon as Raup got back to him, so we could tell Raup about the idea together. 

 

            "Don't bother" I diffidently replied, "I have some things to do.  I'm sure you can explain it to him without my help."  Alvarez had switched sides so quickly that I couldn't tell whether I had won the argument or not.

 

            A short while later Alvarez came to my office.  "Raup just called," he said.  "I told him that I had a young colleague named Rich Muller had just invented a model that could explain his periodicity.  Then I told him about your idea that the sun has a companion that orbits it with a 26 million year period."  I was flattered that the great Luis Alvarez was describing my theory to the great David Raup, but also a little worried about my off-the-cuff idea being presented to an eminent scientist that I hadn't even met.  "Raup said that someone else had invented that idea last week.  Raup told Gene Shoemaker about it, and Gene said the orbit of the star is unstable.  Your idea doesn't work."